Photolysis of [M(CO)(6)] (M = Cr, W) in the presence of BH3.L (L = NMe3, PMe3, PPh3) gave isolable borane complexes [M(CO)(5)(eta(1)-BH3.L)] (1a, M = Cr, L = PMe3; 1b, M = Cr, L = PPh3; I1, M = Cr, L = NMe3; 2a, M = W, L = PhMe3; 2b, M = W, L = PPh3; 2c, M = W, L = NMe3). In products 1 and 2, the monoborane-Lewis base adduct coordinates to the metal center through a B-H-M three-center two-electron bond, which was confirmed by X-ray structural analyses of 1a, 2a, and 2b at low temperature. The X-ray crystal structural analysis of Ic at ambient temperature also showed the same coordination mode, although the positions of hydrogen atoms on the boron were not determined. The H-1 NMR spectra of 1 and 2 exhibit only one BH signal at -2 to -3 ppm with an intensity of 3H in the temperature range of -80 degrees C to room temperature. This indicates that the coordinated BH and terminal BH's are rapidly exchanging in solution even at low temperature. When [Mo(CO)(6)] was used as a precursor, the formation of the corresponding molybdenum-borane complexes, [Mo(CO)(5)(eta(1)-BH3.L)] (3a, L = PMe3; 3b, L = PMe3; 3c, L = NMe3), was observed by NMR spectroscopy, but the complexes could not be isolated because of their thermal instability. Complexes of pyridineborane [M(CO)(5)(eta(1)-BH3.NC5H5)] (1d, M = Cr; 2d, M = W) were also observable by NMR spectroscopy. Fenske-Hall MO calculations for the model compound [Cr(CO)(5)(eta(1)-BH3,PH3)] (1e) demonstrated that the bonding between the borane and metal can be described as donation of the bonding electron pair of BH to the al orbital of [Cr(CO)(5)], and that pi back-donation from the metal d orbital to the antibonding o* orbital of BH is negligible. Compounds 1-3 can be regarded as model compounds of the methane complex [M(CO)(5)(CH4)], which is observed in the photolyses of [M(CO)(6)] in methane matrixes. Structural and spectroscopic features of the ligated borane are discussed and compared with those of related compounds.